A solid-state image pickup device intended to be used for oral radiography has been disclosed in Patent Document 1. For such application, the period of incidence of X-rays to be imaged is considerably short so that the solid-state image pickup device must capture the timing of incidence of X-rays to image the X-rays. Therefore, the solid-state image pickup device disclosed in Patent Document 1 also includes a triggering photodiode for detecting an incidence of X-rays separately from an imaging photodetecting section including a plurality of photodiodes arrayed two dimensionally in order to take an X-ray image. And, this solid-state image pickup device detects an incidence of X-rays by monitoring an electrical signal outputted from the triggering photodiode and then obtains an X-ray image based on an electrical signal outputted from the imaging photodetecting section.    Patent Document 1: Japanese Translation of International Application (Kohyo) No. 2002-505002